The present invention relates to an AC or DC power transmission system, a method of measuring a voltage and a method of calibrating a voltage measurement system.
The power industry has a need for monitoring the condition of power transmission systems. For these purposes, it has been known to make current measurements using only optical technology. One example of such optical current sensors utilizing the Faraday effect is described in the applicant's own international application WO/2004/099798. In a Faraday effect current sensor, the polarization plane of a polarized incident light undergoes a rotation, which is a function of the magnetic field created by the electric current to be measured. Such Faraday effect current sensors have the advantage over generally known Rogowski coils and similar metallic current sensors that they may be constructed entirely from dielectric materials and may thus be applied in locations where a very high electric field is present.
It has also been known to carry out voltage measurements using an optical sensor. This may be achieved by utilizing the Pockels effect, which is an optical effect in anisotropic crystals. In a voltage sensor utilizing the Pockels effect, the polarization plane of incident light passing through the crystal undergoes a rotation if there is an electric field applied over it. The main principle of such Pockels effect voltage sensors thus resembles the principle of the above mentioned Faraday effect current sensor, namely that the induced electric field over the sensor element gives rise to a small variation in the polarization of the light going through the sensor. This variation can be measured and from such measurements the electric field strength may be derived. From the derived electric field strength at the location of the sensor, the voltage on the wire may be determined.
There are several advantages of using an optical voltage sensor, the first being simplicity. The optical voltage sensor is comprised of few parts and hence is easy to assemble. Further, the measured signal is solely optical so that there is no electrical noise induced in the measurement. Yet further, there is no electrical connection between the conductor to be measured and the ground like in a conventional voltage divider. Such electrical connection may cause problems such as a short circuit.
The physics behind the optical voltage sensor is based on the Pockels effect, which was discovered in the late 19th century. It has since been used in various known optical devices such as Q-switches and Chirped pulse amplification. The effect is expressed in the linear term of the following equation:
      1    n    =            1              n        0              +    rE    +          RE      2      wherein E is the electric field. n, n0, r and R are all tensors, respectively describing the refractive index, the ordinary refractive index, the linear and quadratic electro-optic coefficient. If E is applied correctly with respect to the r tensor (the crystal) and the quadratic term is neglected, n will become non-symmetric, thus giving rise to birefringence. This means that light sees a different refractive index depending on the orientation of the polarization with respect to the r tensor.
In known optical voltage sensors, the crystals which exhibit the Pockels effect have electrodes attached to them and have a predetermined trajectory for light passing through. The above configuration is generally known as a Pockels cell and functions as a voltage-controlled wave plate. Such configurations are used in various prior art publications. One example includes an IEEE publication titled “230 kV Optical Voltage Transducer Using a Distributed Optical Electric Field Sensor System” by P. P. Chavez, F. Rahmatian and N. A. F. Jaeger. The proposed sensor system uses a Pockels effect crystal located within an insulating section between line voltage and ground. The full line voltage thus is applied over the Pockels cell, which at least for medium voltage and above requires a high insulation level.
U.S. Pat. No. 6,285,182 discloses an electro-optic voltage sensor having no need for a ground reference. However, the voltage sensor still needs metallic electrodes in the vicinity of the Pockels crystal. EP 0338542 discloses a similar electro-optic voltage sensor using a Pockels sensor and capacitive voltage divider located within a common housing. Thus, only AC voltage is measureable.
Further prior art describing the use of Pockels cells voltage sensors located within an insulating section for measuring the voltage on high voltage lines, or similar technologies, are among others: U.S. Pat. Nos. 6,380,725, 5,029,273, 5,635,831, 6,388,434, 6,946,827, 6,411,077, JP 10132864, WO2009/138120, U.S. Pat. Nos. 4,269,483, 6,492,800, 7,769,250, 7,057,792, 6,353,494, JP 2005315815, JP 03044563, WO00/13033, EP 0011110, U.S. Pat. No. 4,253,061, WO98/13698, CA 2,289,736 and GB 1353543.
Using a conventional Pockels cell configuration as described above has the disadvantage that metallic electrodes need to be attached adjacent the crystal within the voltage sensor. For high voltage or medium voltage purposes, this necessitates a large amount of insulation, resulting in a very large voltage sensor. Further, since metallic objects are located wihtin a high electric field, there is a risk of insulation failure and a dielectric breakdown within the voltage sensor. Such dielectric failures would result in the immediate failure of the voltage sensor and possibly in an interruption of the power transmission system. It would therefore be an advantage to have a voltage sensor with no electrodes attached to the crystal. Thus, it is the object of the present invention to provide methods and systems for measuring the voltage of a conductor without the involvement of any metallic materials other than the conductor itself.